This invention relates to holography, and in particular to a holographic memory and a holographic recording system. A holographic memory makes use of a memory medium upon which many individual holograms are stored. Each hologram represents a different bit pattern or "page". Information is stored by directing an information beam and a reference beam to a desired location on the memory medium. The information beam, which contains a bit pattern formed by a page composer, interferes with the reference beam to form the hologram. To read out the information, a readout beam selectively illuminates one of the holograms stored, thereby producing a reconstructed image of the bit pattern stored in the hologram. An array of photodetectors is positioned to detect the individual bits of the reconstructed bit pattern.
The holographic memory is an extremely attractive form of mass memory. In a "bit-by-bit" type of optical memory, a single recorded spot on the memory medium represents only one information bit. In a holographic memory, on the other hand, a single hologram on the same memory medium may represent a page containing as many as 10.sup.5 bits. Memories having 10.sup.5 or 10.sup.6 pages have been proposed, with each page containing about 10.sup.5 bits.
Another advantage of the holographic memory is that the information stored in the hologram is stored uniformly throughout the hologram rather than in discrete areas. The hologram is thus relatively insensitive to blemishes or dirt on the memory medium. A small blemish or dust particle on the memory medium cannot obscure a bit of digital data as it can in a bit-by-bit memory.
Despite substantial research efforts over the last several years, holographic memories have not yet achieved their potential. Development of a holographic memory having a memory capacity of 10.sup.10 to 10.sup.11 bits has been hampered by limitations in the performance of some of the components of the holographic memory. In particular, difficulties have been encountered with the light beam deflectors and page composers required for a high capacity holographic memory. Present day light beam deflection technology is not capable of providing the large number of resolvable beam positions at access rates which are acceptable for a large scale holographic memory. In addition, a satisfactory page composer capable of producing a two dimensional array of 10.sup.4 or 10.sup.5 bits has not yet been developed. Such a large scale page composer presents difficult problems in addressing various bits. If row by row addressing is used to shorten the composing or setup time of the array of bits, the optically active page composer material must also exhibit memory characteristics. A page composer meeting these various conflicting requirements has presented a challenge to researchers.